Shafts are critical to the construction and operation of tunnels of all types. They enable access from the ground surface to the tunnel level, for all tunnel types, including giving passengers access to mass transit and underground rail tunnels. They act as drop shafts for wastewater tunnels and downtake and uptake shafts for water supply tunnels, as well as inlet and outlet structures for flood control tunnels and dams. Shafts also provide ventilation for highways, mass transit, and rail tunnels.
Shafts generally are circular or elliptical in section for structural efficiency reasons, although more complex geometries may be appropriate for particular shafts. Shafts are designed to resist lateral pressures during construction and throughout their service lives. These include water, earth, and rock pressures.
Balancing earth, water, and rock pressure
Water pressures in free groundwater conditions are assumed to vary linearly with depth. This rule, however, does not apply when shafts penetrate artesian aquifers. Likewise, earth pressures generally are assumed to vary linearly with depth, although for deep shafts in soil, a limiting pressure is often assumed. Rock pressures for shaft design are developed on the basis of rock structure.
Earth and water pressure are assumed to be uniform around the shaft perimeter, while rock pressures may vary. Uniform earth and water pressure results in uniform compressive forces in the shaft lining. Variable rock loads around the shaft perimeter will result in development of bending moments within the shaft lining. Secondary moments related to load eccentricity are often assumed for shafts constructed in earth.
Shafts excavated in rock are supported during construction by rock reinforcement and rock surface protection, either in the form of welded wire fabric or shotcrete (sprayed concrete). This construction support generally is not assumed to provide permanent support. If the rock reinforcement is installed with double corrosion protection, it can be considered permanent and the final shaft lining then can be designed for water pressure only. However, installation of rock reinforcement with double corrosion protection will delay excavation progress and may increase overall construction costs.
Generally, shafts are constructed from the ground surface down, using conventional shaft sinking methods. However, in rock tunnelling projects, shafts not used for primary construction access can be constructed using raise boring. In raise boring, a mall pilot hole is drilled from the ground surface to tunnel depth. A reaming bit is then used to excavate the shaft to final dimensions.
A number of methods are used to support shafts excavated in soil during construction. Some support systems are installed from the ground surface prior to the start of excavation. Others are installed concurrently with excavation.
Supporting shaft construction
Some forms of support are installed from the ground surface prior to the start of excavation, including steel sheet pile, slurry walls, jet grout walls, secant pile walls, and ground freezing. Other segmental lining support systems are installed concurrently with excavation, including steel liner plate, ductile iron segments, and precast concrete segments.
A hybrid type of excavation support would be soldier piles and lagging. In this method, the soldier piles are installed prior to the start of excavation and then timber or concrete lagging is installed concurrently with excavation. Dewatering is often required when segmental lining or soldier pile and lagging construction is used.
For certain shaft applications, ductile iron segments and precast concrete segments can be used as the final lining. When the other excavation support methods are used, a cast-in- place concrete final lining generally is required. In some cases, a shotcrete (sprayed concrete) lining can be substituted for the cast-in- place concrete lining.
WSP in action
In Washington, DC, WSP was a part of the Design-Build team for the First Street Tunnel, a storm water storage tunnel built to prevent flooding as a part of DC Water’s Clean Rivers Project. During a storm event, the FST will store up to 8 million gallons of combined sewer overflow (CSO) in its 2,800 feet of tunnel and 4 shafts. It will utilise a pump station located at Rhode Island Avenue to pump storm water back to the shallow sewer system, once the storm has subsided. The project’s use of a centralised freeze plant to construct support of excavations for shafts, adits, and adit-to-tunnel junctions was a first. A utility trench was constructed along a back alley to provide brine supply to the three satellite sites.